Processes which allow a pure substance to polymerize are well known in the state of the art. Initiators capable of generating free radicals, either by thermal decomposition or photo-induced, is a typical way of initiating polymerization. The process may also be induced via cationic polymerization, which takes place in acidic solution, or using Ziegler-Natta catalysts which not only induce polymerization but also accommodate the monomer units in preferred positions thus generating stereoregularity in polymer chain such as tacticity. Among the polymerization in solution, the electrochemical polymerization is a well-known procedure, which considers the application of an electric potential in monomers of a preferred structure such as pyrrole and various anilines, soluble in the solvents to be used, but using salt solutions acting as a supporting electrolyte.
There are not many examples of technologies related to the production of polymers using an electric field without initiators or electrolyte solutions. Some patents described relief made on a surface by using complex systems of multifunctional monomers which also possess good adhesion. The first patent related to the application of an electric field on a mixture of monomers was issued on 1941 (W. Luster, U.S. Pat. No. 2,257,177). This process is performed under reduced pressure (0.001 and 100 mm Hg, preferably 10 to 40 mm Hg), and differs of voltolisation (described below) as the author explain, because they consider conducting a chemical reaction by applying an electric shock of high frequency, at least 1 megacycle and up to 600 megacycles per second. In voltolisation, frequencies below 10,000 cycles are used. Thus, W. Luster indicates that it is possible to polymerize, dehydrogenate or perform addition reactions using an electric field.
Patent GB1205438 (16 Sep. 1970) of Xerox is probably the most general of all, because it describes a polymerization between electrodes (again in a multi-functional monomers mixture) in the absence of chemical initiators, catalysts and electrolytes, only by applying an electric field. This polymerization process is considered as charge injection polymerization. However, the examples are related to complex multifunctional systems which generate highly cross-linked systems with good adhesion in plates for printing or for producing relief images of Braille type. This work is similar to later inventions described in U.S. Pat. No. 3,879,275 (Apr. 22, 1975), U.S. Pat. No. 3,862,841 (Jan. 28, 1975) and U.S. Pat. No. 3,965,276 (Jun. 22, 1976).
Under the C08F2/52 classification, according to International Patent Classification IPC, polymerization processes by electric discharge or voltolisation are described. It is interesting to notice that under this classification some documents described the so-called electro-polymerization processes. However, going into the content of such documents, it is evident that such processes correspond to electrochemical polymerization, being the corresponding independent claim for the process not clear to specify this type of polymerization, (see e.g. U.S. patent 2010048815 or WO2008039151). The documents described under this classification can be grouped into sub-categories. The plasma polymerization, where monomer is in the gaseous state and the polymerization is initiated at the plasma; plasma polymerization using a photo-initiator; a monomer grafted into a polymer, activated by gamma radiation, X-ray, etc; radio-polymerization by corona discharge or ion beam, used mostly for the formation of conducting polymers, and so on.
Summarizing, none of these technologies has been applied in isolated monofunctional molecules in the isotropic state or in a mesophase, under normal conditions of pressure and DC fields. The two major types of inventions are first originated early in the preparation of crosslinked films for relief, where mixtures of polyfunctional monomers were used. The second, which is most currently in uses, refers to polymerization in plasma, corona discharge or charge injection, but in gas or plasma state.
In the field of liquid crystals (LC), various types of technologies requiring the preparation of polymers mixed with LC possess attractive features for use in electro-optics, such as polymer-dispersed LC (PDLC) as described in US2006187402, WO2006061808, US2006119917, WO0150200, JP11223810, KR100257886, among others; polymer stabilized LC (PSLC) as in JP2005331757, CN1811528, U.S. Pat. No. 6,215,543, U.S. Pat. No. 5,504,600 and U.S. Pat. No. 6,075,583, WO2010032962 among others. All these documents take into account the preparation of the polymer both using mostly photo-initiators, or free radicals initiators, and in rare cases initiated as thermal polymerization.
For LC technology, the general purpose when using an electric field is to produce a better alignment of liquid crystal system, as described in CA2470493, US2006256258, JP2002031821.
In the development of liquid crystal displays (LCD) a preferential alignment of the LC to be used as optical switch is required. Actual technology achieve homogeneous alignment using thin layers of i.e. polyamide, as a few layers on the electrodes, on which grooves are induced by mechanical processes and also using photo-aligned polymers capable to be photo-isomerized. In these grooves the LC molecules are arranged so as to obtain a pre-ordering required in the pixel. In order to obtain homeotropic alignment substrates coated with hydrophobic films such as silane compounds are required (Alignment Technologies and Applications of Liquid Crystal Devices, Kohki Takatoh et. al., 2005, Tayor & Francis, ISBN 0-748-40902-5). With this technology it is possible to obtain planar alignment and also homeotropic alignment, depending on the desired end goal. A pixel is known as the smallest homogeneous surface which comprises an image and that is defined by its brightness and color.
In the document WO 2008/145297, a liquid crystal mixture composed of materials with negative dielectric anisotropy for use in vertical alignment technology (VA) stabilized by a polymer (PS-VA) is described. The principle of electrically controlled birefringence known as ECB was first described in 1971 (M F Schieckel and K. Fahrenschon, “Deformation of nematic liquid crystals with vertical orientation in electric fields”, Appl. Phys Lett. 19. (1971), 3912), followed by the works of J F Kahn (Appl. Phys Lett. 20. (1972), 1193) and G. Labrunie (J. Appl. Phys. 44 (1973), 4869). For a mixture to be used in LCDs based on the ECB effect, the liquid crystalline phase should have high ratio of elastic constant values K1/K3, high optical anisotropy Δn and a dielectric anisotropy Δ∈ values ranging between −0.5 and −5. J. Robert and F. Clerc (SID 80 Techn. Papers, 1980, 30), J. Duchene (Displays 7, 1986, 3) and H. Schad (SID 82 Digest Techn. Papers, 1982, 244). The electro-optical elements based on the ECB effect must possess homeotropic alignment. The patent WO 2008/145297 refers to a crystalline liquid medium based on a mixture of polar compounds with dielectric anisotropy (Δ∈) negative in order to be used in vertical alignment technology (VA).
Mixtures of liquid crystal phases mostly with nematic, used in PS-VA, such as those described in WO 2008/145297 mentioned above, are currently the market leading technology for the development of high quality displays. Here the homeotropic orientation of the mesophase is used in the electro-optical element. They replace the conventional technology of vertical alignment (VA) developed before by the industry of flat displays. This new technology is based on the formation of polymer networks or polymer layers within the pixel where the molecules with negative dielectric anisotropy can turn against an applied field switching on and off the light. The local tilt predefines the change in direction of liquid crystal, resulting in very fast switching times, one of the relevant parameters today for the development of liquid crystal flat panel displays. The development of such systems showing higher efficiency allows for better image quality in motion, lower switching times, higher contrast and better light transmission, which also implies a decrease in the required power backlight.
Typically the manufactures of displays which use PS-VA technology necessarily require the use of radical initiators in polymerization as photo initiators and hence also the use of polarized UV light to form the networks of polymers.
The ferroelectric liquid crystals (FLC) are potential candidates to improve the existing technology of flat panel displays, as they present response times up to 500,000 times faster than currently used systems based on nematic mesophases. Clark and Lagerwall in the document EP0032362 describe an electro-optical apparatus where a FLC is included into a cell or pixel, and due to surface stabilization, a switching occurs (SSFLC). However, the problems of defects in the formed mesophase and others issues as the mesophase stabilization, do not allow the use of this technology in large screens, problems apparently already partially solved. These FLC systems are still conditioned to the cell thicknesses which must be comparable to the material pitch, which usually does not exceed 2 microns.
The PS-FLC mode for displays considers the preparation of a network similar to that for PS-VA in nematics, but now applied to smectic-induced mixtures with planar orientation, where the FLC has a V-shaped switching (S. Kawamoto, et. Al., Displays, 25, 1, 2004, 45-47). The authors suggest that this technology can also be developed in both, smectic A and smectic C phase in the presence of an FLC material. Here again, the polymerization is conducted using a photo-initiator which forms the polymer network, where the amount of polymerizable material is 6% on the total mixture.
In summary, there are different procedures for carrying out polymerization processes, being one more effective than others, all achieving the same objective which is to cure the polymer.
However, still remains a need to carry out polymerization processes which enable the same or better results but which can be easier to be implemented and more effective than the existing ones.
The present invention proposes a new technology based on liquid crystals, which considers the use of electric field to promote in situ polymerization and thus getting highly ordered systems where guest molecules can introduce new properties into the matrix which is formed. This implies a major advantage over the prior art, since it does synergistically align a molecule while simultaneously, an in situ polymerization induced by the field is performed. As already discussed, both, for display's development, optical switches or polarizer, the electric field has been always used only for molecular orientation, but never to build the polymer matrix.
Attended the above, is an objective of the present invention to produce polymers without the use of polymerization initiators as photo- or radical initiators and without the use of solvents. Preferably, the compound to be polymerized possesses liquid crystal properties, so the reaction can be performed either in one of the mesophases or at the isotropic state.
It is another object of the invention that the so formed polymer imparts a preferential ordering, thus allowing to produce the alignment of the liquid crystal or the rest of the components of a selected mixture.
Another objective of the present invention is that due to the order achieved in the polymer matrix its physical properties may be changed by the proper selection of the second component in the mixture, where the physical properties can be so different as ferroelectric, magnetic, fluorescent or nonlinear optical, to name a few.
One of the preferred embodiments of the invention is the construction of a pixel or polarizer to switch light obtained by in situ polymerization of one or more polymerizable compound by the process described below, where between the electrodes a single polymeric material and between 0 and 90% of a second organic or inorganic component, or a mixture of components, which previously haven't undergone any decomposition or structural changes within the cell by any pretreatment.
Finally, it is an object of the invention that the preferably uses given to the device should be as an electro-optic switch, or polarizer or a pixel in a liquid crystal display.